1. Field of the Invention
The invention relates to the field of chemical ionization (CI) and electron impact ionization (EI) mass spectrometry. In this field it is regarded as advantageous to use a single ion source which can be operated in either the CI or EI mode. By means of apparatus and methods disclosed herein a sample material in a single ion source is analyzed effectively simultaneously in CI and EI modes, the CI and EI mass spectra being separated by electronic means to provide simultaneous display of the two types of mass spectra.
2. Discussion of the Prior Art
In the prior art of chemical ionization (CI) mass spectrometry and electron impact ionization (EI) mass spectrometry it has been regarded as useful to construct ion sources which, by means of mechanical and electrical changes, operate in either the EI or CI mode. It is considered advantageous to change from one mode to the other in as short a time as possible, for with a rapid changeover it becomes possible to examine both the CI and EI spectra of transient sample materials as obtained by thermal evolution of a heated sample or as observed in the effluent of a gas chromatograph or liquid chromatograph. Such devices employ one ionization chamber which is operated in either the CI or EI modes, and one filament as the electron supply for either mode of operation.
Past commercial practice has been to use a mechanical linkage for changing the sizes of the required apertures for electron entry and ion exit, and to provide electrical means for the required changes in electron energy and ion optical parameters, such changes being accomplished without venting the vacuum system. Such past practice has developed to a state whereby the changeover is accomplished within several seconds. Problems arise in attempting to reduce the time further because of the relatively slow nature of even the fastest mechanical motions, and the fact that when only one ionization chamber is used for both CI and EI modes the ionization chamber must be filled with reagent gas when switching from EI to CI and emptied when switching from CI to EI. A subsidiary complication is that the reagent gas valve must be actuated in concert with the required electrical and mechanical changes.